Abstract : Electro-absorptive long wavelength (Lambda = 1550 nm) semiconductor modulators with low drive voltage are indispensable components for future high bit-rate communication networks. Recently, Wannier-Stark Localization (WSL) in an InGaAs/InAlAs superlattice was employed to achieve sub-volt operation combined with high extinction. However, such devices are not readily integrable with the existing high quality InGaAsP/InP lasers. In this work, we demonstrate for the first time electro-absorptive optical modulation by WSL in an InGaAs/InGaAsP superlattice. The WSL sample is grown as a p-i-n heterostructure on an n+-InP substrate by low pressure (2x103 Pa) metalorganic vapor phase epitaxy. The intrinsic region consists of five Wannier superlattices each of them with five periods of quantum well and barrier layers. Moreover, quaternary spacer layers were employed to improve the quality of epitaxial growth. Superlattice formation was verified by the observation of satellite peaks in the double crystal x-ray diffraction rocking curves. Photoluminescence peak (300 K) for the Wannier superlattice was seen at 1450 nm without any traces of ternary signal.